Technical field The present invention relates to a clamping device, particularly for wheels that are subjected to surface finishing operations, according to the preamble to main Claim 1. A further subject of the invention is a method according to the preamble to Claim 9.

Technological background The invention relates particularly but not exclusively to the specific technical field of machining operations, for example, turning or other surface finishing operations on light-alloy, particularly aluminium-alloy, vehicle wheels.

In this field, the semi-finished alloy wheel is subjected to cycles of machining by the removal of shavings, mainly to turning cycles, after the semi-finished wheel has been arranged and clamped on a piece-holder device, for example, provided on a rotary chuck. The machining cycles may require the piece to be positioned on the chuck several times, according to the number and geometrical complexity of the surfaces to. be machined.

A clamping device of the type indicated is known from international application WO 02/098589. In the device described therein, the semi-finished wheel is clamped to the rotary chuck by the fitting and reversible clamping of a plurality of expansion pins in the holes in the wheel which are intended for the fixing of the wheel to the vehicle wheel-hub.

A solution of this type offers the possibility of limiting the number of times the wheel is positioned on the chuck in order to machine its surface completely, but involves the disadvantage of rendering the wheel subject to vibrations which are generated during the machining operations and which are amplified as a result of the distances that exist between the most lateral wheel regions which are subjected to machining and the face region which is clamped to the chuck. In this connection, the known clamping device further provides a vibration-damping ring for resiliently contacting an outer circumferential region of the face portion of the wheel.

However, with particularly heavy machining operations, for example, those that are performed with machining inserts of large dimensions such as those having a so-called"biscuit"configuration, with cutter radii of the order of a few millimetres, the known damping elements are inadequate and the magnitude of the vibrations induced during machining operations may lead to the risk that the quality of the surface finishing of the wheel may be compromised and the life of the insert may be reduced, with a consequent increase in the times and costs of the machining cycles required.

Description of the invention The problem underlying the present invention is that of providing a _device and a method for clamping pieces that are subjected to machining operations, in particular for clamping wheels during surface-finishing operations, which are designed structurally and functionally to overcome the limitations discussed with reference to the prior art mentioned.

This problem is solved by the invention by means of a device and a method according to the appended claims.

Brief description of the drawings Further characteristics and advantages of the invention will become clearer from the following detailed description of a preferred embodiment thereof which is described by way of non-limiting example with reference to the appended drawings, in which: - Figure 1 is a partially-sectioned, schematic front elevational view of a device formed in accordance with the invention, - Figure 2 is a plan view of the device of Figure 1, from above, and - Figure 3 is a schematic view showing a detail of the device of the previous drawings, on an enlarged scale.

Preferred embodiment of the invention With reference to the drawings mentioned, a device for clamping pieces that are subjected to machining operations to a piece-holder table 2, for example, provided with a chuck 3 rotating about an axis X, is generally indicated 1.

The device 1 is intended to be incorporated in a work centre which, however, is not shown, but in which one or more tools are operated in

association with the chuck 3 in order to perform the machining of the piece (for example, surface turning) in accordance with the work cycles required.

Moreover, although the device of the invention is described below with particular reference to the clamping of semi-finished light-alloy vehicle wheels 4, this illustration should not be understood in limiting terms, although the example described represents a preferred selection for the application of the invention. The device 1 is therefore equally usable in all applications in which it is necessary to perform surface machining operations on substantially axially symmetrical solids with particularly complex geometry of the surfaces to be machined, such as those which distinguish vehicle wheels.

In this context, the term"wheel"is also intended to denote the wheel/rim structure as a whole, which is typically provided with a drilled face portion 5 (conventionally having holes 6 for fixing to the wheel hub by means of bolts), from which a substantially cylindrical wall 7 extends transversely, constituting the channel for the fitting of the tyre.

Pin clamping means 8, which cooperate with the holes 6 in the wheel and the characteristics of which do not constitute a subject of the present invention, are provided for clamping the wheel 4 to the piece-holder table.

Any clamping device arranged for engaging with reversible coupling in the (preselected number of) holes 6 in the wheel may advantageously be provided, so as to clamp the wheel to the piece-holder table in a central face region 5 of the wheel. Typically, five pin clamping elements are provided (only one of which is shown in Figure 1) for reversibly engaging corresponding holes 6 provided in the wheel.

According to a principal characteristic of the invention, the device 1 further comprises means, generally indicated 9, for the localized support of the wheel 4 in an outer, peripheral, circumferential region, indicated 10, of the face portion 5.

The support means 9 comprise a plurality of pillar support elements 11 which can be locked selectively to constitute a rigid prop for supporting the wheel 4 during the machining operations, as will become clear from the following description.

More particularly, the pillar elements 11 are resiliently yielding relative to the piece-holder table 2 in a direction parallel to the axis X, for example, by virtue of resilient springing 12, and each element can be locked selectively relative to the table 2 in a preselected position, for example, by means of a clamping device 13 of the pillar 11, shown schematically on an enlarged scale in Figure 3.

The pillar elements 11 are arranged at regular intervals circumferentially (there are three pillar elements in the embodiment described) in the peripheral region 10 of the wheel so as to ensure localized support of the wheel in the outer, lateral region. They are also operatively independent of one another so as to support the wheel even when the points of contact of the wheel on the respective pillars are not coplanar.

In operation, once the wheel 4 has been clamped to the table 2 by the pin clamping means 8, a relative approach movement between the wheel 4 and the support means 9 brings the peripheral region-10 of the wheel into contact with the pillars 11. Owing to the resilient yielding characteristics of the pillars 11, bearing contact of each pillar is ensured even when there are errors in the coplanar condition of the surfaces of the semi-finished wheel.

Once each pillar 11 has been brought to bear on the corresponding wheel portion 4, it is locked (by means of the device 13) relative to the table 2, thus constituting a localized, unidirectional, rigid prop for the wheel. In this operative condition, a rigid supporting effect on the wheel is achieved in a lateral region of the face portion 5 and is added to the central support achieved in the region of the holes 6. It should be noted that this clamping and supporting effect on the wheel is ensured, irrespective of geometrical imperfections and of dimensional tolerances in the semi-finished wheel that is subjected to machining.

According to the invention, the device 1 may further comprise counter- clamping means, generally indicated 14, cooperating with the support means 9 for the selective restrained clamping of the wheel 4 in the region 10 of contact with the corresponding pillar 11. The counter-clamping means 14 comprise respective brackets 15 which are articulated on the table 2 so as to be pivotable about an axis arranged parallel to the axis X, away from and

towards an operative position of engagement on the wheel 4 in order to exert, with a bracket end 15a, a clamping pressure in opposition to the reaction exerted by the rigid restraint of the corresponding pillar 11 (when it is locked relative to the table 5).

When the device is in operation, once the wheel 4 is supported rigidly by the pillar elements 11, the brackets 15 are moved to the operative position of engagement on the wheel in the region of an edge 10a of the peripheral region 10 thereof, in order to achieve localized, restrained clamping in the region of the rigid support offered by the respective pillar 11. The lateral wheel clamping action thus achieved advantageously prevents the generation of vibration phenomena induced in the wheel, in particular during machining of the opposed-surfaces of the wall 7, where there are the greatest distances (that is, the longest lever arms) between the points of contact of the machining tool and the central face region of the wheel.- - The invention thus solves the problem posed, affording many advantages over known solutions.

A first advantage lies in the fact that, with the device for clamping pieces during machining operations according to the invention, an improved quality is achieved in the surface finishing of the piece.

Another advantage is that it is possible to count on greater endurance and longer life of the inserts of the machining tools as well as the possibility of using tools with larger inserts or with greater advance parameters, without the risk of inducing vibratory phenomena which might compromise the final quality of the piece.

Yet another advantage is that it is possible to machine the piece with a single positioning on the piece-holder table, in a first stage with brackets engaged (shown in continuous outline in Figures 1 and 2) to complete the machining of the upper wheel portion (the cylindrical wall for coupling with the tyre), and in a subsequent, second stage with brackets disengaged (shown in broken outline in Figures 1 and 2) to afford access for the surface machining of the lower wheel portion (the face portion).